The nuclear power industry long has been engaged in a multitude of studies and investigations seeking improvement in the stamina and reliability of the materials and components forming a reactor based power system. One such investigation has been concerned with intergranular stress corrosion cracking (IGSCC) which heretofore principally has been manifested in the water recirculation piping systems external to the radiation intense reactor core regions of nuclear facilities. Typically, the piping architecture of these external systems is formed of a stainless steel material. Generally, these studies have determined that three factors must occur in coincidence to create IGSCC promotional conditions. These factors are: (a) a sensitization of the metal (stainless steel), for example, such as caused by a chromium depletion at grain boundaries which may be caused by heat treatment in the course of normal processing of the material or by welding and like procedures; (b) the presence of tensile stress in the material; and (c) the oxygenated normal water chemistry (NWC) environment typically present in a boiling water reactor (BWR). This latter environment is occasioned by any of a variety of oxidizing species contributed by impurities in reactor coolant water. By removing any one of these three factors, the IGSCC phenomenon is essentially obviated. Such removal particularly has been accomplished with respect to the latter, oxygenated environment factor through employment of an electrochemical potential monitoring approach combined with an associated hydrogen water chemistry (HWC) technique providing for a controlled addition or injection of hydrogen into the aqueous coolant environment.
Electrochemical potential monitoring is carried out employing paired electrochemical half-cell probes or electrodes which are mounted within the recirculation piping or in an external vessel which has its water source from the reactor water in the recirculation piping. The electrodes are accessed to the external environment through gland type mountings or the like. Where, as in the instant application, the electrode system of interest involves the potential from a metal corrosion electrode, then the reference electrode can conveniently be a metal-insoluble salt electrode, if the metal-salt couple is chemically stable and if appropriate thermodynamic data is available. Accordingly, one of the thus-mounted probes which is configured as a reference electrode may be based, for example, on a silver/silver chloride half-cell reaction. Once the reference electrode half-cell is defined, the cell is completed with the sensing cell portion based upon a metal such as platinum or stainless steel. Verification of the reference electrode and/or the electrode pair is carried out by thermodynamic evaluation and appropriate Nernst based electrochemical calculations in combination with laboratory testing within a known environment.
Half cell electrodes developed for use in reactor circulation piping traditionally have been configured with metal housings, high temperature ceramics, and polymeric seals such as Teflon brand polytetrafluoroethylene. However, most, if not all of these structures, have limited lives, especially in HWC, due to passage of H.sub.2 to the active portion of the sensor where chemical and spurious electrochemical reactions occur. Such reactions cause significant voltage shifts. One such prior reference electrode design is set forth in U.S. Pat. Nos. 4,500,413 and 4,576,667. This electrode design, however, has not proven useful in use in BWRs or in high temperature test systems.